Recently, due to decreases in the size of electronic equipment and increases in the speed of electrical signals, electronic parts used in electronic equipment are becoming smaller in size and multifunctional. Examples of electronic parts which have become smaller in size and multifunctional are BGAs (ball grid arrays), CSPs (chip size packages), and MCMs (multichip modules) (collectively referred to below as BGAs). BGAs have electrodes disposed in a lattice-like pattern on the rear surface of a BGA substrate.
In order to mount a BGA on a printed circuit board, electrodes of the BGA and lands of the printed circuit board are connected with each other by solder. However, supplying solder to each electrode and carrying out soldering requires a great deal of effort, and solder cannot be supplied from the exterior to electrodes located at the center of a substrate. Therefore, a method is used in which solder is previously mounded up on the electrodes of a BGA. This method is referred to as solder bump formation.
Solder balls, solder paste, and the like are used for solder bump formation on BGAs. When forming solder bumps with solder balls, a sticky flux is applied to the electrodes of a BGA, and solder balls are placed on the electrodes to which the flux was applied. The BGA substrate is then heated by a heating apparatus such as a reflow furnace to melt the solder balls and form solder bumps on the electrodes. Substrates for semiconductors such as BGA substrates will be collectively referred to as module substrates.
When forming solder bumps on the lands of a wafer using a solder paste, a metal mask having holes which are bored in locations matching the lands of the wafer and which have about the same size as the lands is placed on the wafer, a solder paste is spread atop the metal mask with a squeegee so that solder paste is applied to the lands of the wafer by printing. The wafer is then heated in a reflow furnace to melt the solder paste and form solder bumps.
With conventional BGAs, solder balls made of a Sn—Pb alloy were used for solder bump formation. Sn—Pb solder balls not only have excellent solderability with respect to the electrodes of a BGA but particularly a Sn—Pb eutectic composition has a melting point which does not have thermal effects on BGAs or printed circuit boards at the time of soldering. Moreover, the solder balls contain soft Pb, impacts are absorbed when electronic parts or electronic equipment using these solder balls is dropped, and this fact greatly contributes to increasing the lifespan of electronic parts and electronic equipment. The use of Pb is now being regulated on a global scale, and a Sn—Pb eutectic composition which has conventionally been used for soldering is also being regulated.
Sn—Ag—Cu based solder alloys such as Sn-3.0Ag-0.5Cu and Sn-4.0Ag-0.5Cu have been used as compositions of lead-free solder balls for BGAs. These lead-free solder alloys have excellent resistance to thermal fatigue, but when portable electronic equipment using solder balls having these solder alloy compositions is dropped, interfacial peeling in which peeling takes place from the bonding interface of the solder balls easily occurs, so resistance to drop impacts is inferior. In order to improve resistance to drop impacts, Sn—Ag—Cu—Ni based solder alloys containing Ni have been developed.
However, these Sn—Ag—Cu or Sn—Ag—Cu—Ni based solder compositions used in solder balls have poor wettability compared to conventional Sn—Pb based solders, and when BGAs are mounted on a printed circuit board using solder paste, solder components of the melted solder balls and solder components of the solder paste do not completely commingle with each other, resulting in the problem of the occurrence of the phenomenon of fusion defects in which a layer of an oxide film of Sn develops between the solder balls and the solder paste. FIG. 1 shows an example of bonding of a BGA to a printed circuit board as an example of the phenomenon of fusion defects. Among solder bumps connecting a BGA part 1 and a mounting substrate 2, solder bump 3 was fused, but a fusion defect developed in solder bump 4. FIG. 2 shows a solder bump formed from a solder ball 5 and a solder paste 6 which experienced a fusion defect after heating for mounting. As can be ascertained from FIG. 3, which is an enlargement of FIG. 2, there is only contact at joint 7, which developed a fusion defect. Therefore, if an external impact is applied, the junction easily fractures. If a fusion defect develops, failure easily takes place when an external impact is applied, such as when electronic equipment on which BGAs are mounted is dropped.
In order to prevent problems due to fusion defects, manufacturers of electronic equipment previously inspect joints which have developed fusion defects by a method such as measuring the resistance of electronic equipment, and printed circuit boards which developed fusion defects are repaired or replaced to obviate malfunctions.
The present applicant disclosed a method of applying a post-flux to electrodes of a module such as a BGA as a method of preventing fusion defects which develop when connecting a module such as a BGA and a printed circuit board (WO 2006/134891 A: Patent Document 1).
Compositions for Sn—Ag—Cu—Ni based solder balls for BGAs and the like which have been disclosed include a lead-free solder alloy comprising (1) Ag: 0.8-2.0%, (2) Cu: 0.05-0.3%, and (3) at least one element selected from In: at least 0.01% and less than 0.1%, Ni: 0.01-0.04%, Co: 0.01-0.05%, and Pt: 0.01-0.1%, and a remainder of Sn (WO 2006/129713 A: Patent Document 2), a lead-free solder alloy characterized by comprising Ag: 1.0-2.0 mass %, Cu: 0.3-1.5 mass %, and a remainder of Sn and unavoidable impurities, and a lead-free solder alloy further containing at least one of Sb: 0.005-1.5 mass %, Zn: 0.05-1.5 mass %, Ni: 0.05-1.5 mass %, and Fe: 0.005-0.5 mass %, with the total content of Sb, Zn, Ni, and Fe being at most 1.5 mass % (JP 2002-239780 A: Patent Document 3), a lead-free solder alloy comprising, in mass %, 0.1-1.5% of Ag, 0.5-0.75% of Cu, Ni in an amount satisfying the relationship 12.5≦Cu/Ni≦100, and a remainder of Sn and unavoidable impurities (WO 2007/081006 A: Patent Document 4), and a lead-free solder alloy comprising Ag: 1.0-2.0 mass %, Cu: 0.3-1.0 mass %, Ni: 0.005-0.10 mass %, and a remainder of Sn and unavoidable impurities (WO 2007/102588 A: Patent Document 5).